X-ray imaging is commonly used in medical diagnosis for revealing conditions of organs or tissues of a patient. Absorption contrast imaging is a simple and common X-ray imaging technique, and it normally produces high-quality images of hard tissues and low-quality images of soft tissues. A different X-ray imaging technique, phase contrast imaging, may produce images of soft tissues with improved quality. At present, phase contrast imaging may be conducted using a phase contrast imaging device, which is expensive and inconvenient. Moreover, the current phase contrast imaging device usually has a small field of view (FOV) compared to a normal X-ray imaging device, and thus produces a phase contrast image of a limited size. Hence, it is desirable to find a way of obtaining a phase contrast image of any desired size without directly using the phase contrast imaging device.